The present invention relates to the field of antenna systems, and more particularly, to passive interference-suppressing antenna systems, and the methods used therein.
Radio Frequency (RF) jamming, i.e., intentional RF interference, occurs when RF power is transmitted so as to interfere with the reception and interpretation of an RF receiving system. Jamming signals may interfere with the operation of receivers such as navigation, communication, and global positioning system (GPS) receivers. In this sense electromagnetic interference generally, and jamming in particular, are considered countermeasures to the intended utilization of RF receiver systems.
A number of techniques have been developed to mitigate the deleterious effects of interference on RF receiver systems, including employment of adaptive antenna systems. Adaptive antenna systems typically measure the received power and/or the signal or carrier to noise ratio. These measurements are utilized to modify the reception pattern of the receiving antenna in order to steer a null, i.e., a minimal reception region, in the direction of a interference source and/or steer a gain maximum in the direction of a desired information signal source.
One example of such an adaptive antenna system counter-countermeasure is referred to as the Controlled Reception Pattern Antenna (CRPA). The CRPA operates by receiving electromagnetic energy and feeding it into an antenna array controller unit. Upon detection of received interference power above a threshold, the antenna array controller adaptively alters the antenna array""s reception pattern to attenuate the gain in the direction of the interference. It does this by applying amplitude and phase weights to the auxiliary elements in the array and repeatedly taking power measurements of the incident interference. If a weight is applied to an element and the received power of the interference (or jamming signal) improves (decreases) the antenna array controller decides that the applied weight was a good choice and the weight is saved. If the received interference power increased, or remained the same, however, the antenna array controller decides that the applied weight was not a good choice and it returns to the previous weight for that element. The Antenna Electronics (AE) unit keeps repeating this process until the received power level has returned to its pre-interference level, which implies that a null has been successfully steered in the direction of the interference.
The GPS Antenna System (GAS) is a next generation adaptive antenna array system that is the follow-on to the CRPA/AE. Beam steering in a GAS is accomplished by maximizing the signal-to-noise, or carrier-to-noise, ratio, which indicates that a maximum in the array""s reception has been steered in the direction of the intended signal source. The GAS is believed to operate much faster than the CRPA/AE since it incorporates modern processors.
Both CRPA/AE and GAS adaptive systems have a number of drawbacks. They are limited to mitigating a maximum of 6 interferers and are ineffective against some types of simple jammers as well as some types of advanced jamming waveforms. Also, the nulls produced are relatively wide and these systems require software modifications to deal with changes in jamming technology.
A system and method for receiving transmitted electromagnetic signals in the presence of interference is disclosed. In one embodiment, the present invention improves the interference resistance of an existing RF antenna receiver system by passively prescreening the electromagnetic energy incident upon the antenna. One embodiment includes physically partitioning the incident electromagnetic RF environment of the protected system into two or more fields of view, or sectors, and evaluating the power level of the RF frequencies of interest incident upon each sector. In one embodiment, the passage of the incident frequencies of interest by a sector to the protected receiver system is prevented whenever the power level of any RF frequency of interest exceeds a given threshold. In this way, the present invention passively mitigates the impact of intentional RF jamming and unintentional RF interference. The physical shape of the antenna, the number of sectors into which the RF environment is portioned, the frequencies of interest and the threshold values are specifically tailored to the requirements of the system being protected.
Consequently, embodiments of the present invention eliminate the requirement for an algorithmic determination of an interferer""s presence, its location and its subsequent elimination. In some embodiments, it operates in a single step, without the requirement for software. Also, narrow nulls may be constructed and their width may be modified by varying the geometry and the number of individual sectors partitioning the RF environment.